In a normal adult the filtered amount (load) of Na+ is approximately 25,000 mEq/day:
Equation 35.1
Filtered load of Na+= (GFR) × (Plasma [Na+ ]) = (180 L/day) × (140 mEq/L) , mEq/day
With a typical diet, less than 1% of this filtered load is excreted in urine (≈140 mEq/day).
Because of the large filtered load of Na+ , small changes in its reabsorption by the nephron can profoundly affect Na+ balance and thus ECFV. For example, an increase in Na+ excretion from 1% gThe percentage of the filtered load excreted in urine is termed fractional excretion. In this example the fractional excretion of Na+ is 140 mEq/day ÷ 25,200 mEq/day = 0.005, or 0.5%.
• Fig. 35.11 Segmental Na+ reabsorption. The percentage of the filtered load of Na+ reabsorbed by each nephron segment is indicated. PT, proximal tubule; TAL, thick ascending limb; DT, distal tubule; CD, cortical collecting duct.
to 3% of the filtered load represents an additional loss of approximately 500 mEq/day of Na+ . Because the ECF Na+ concentration is 140 mEq/L, such a Na+ loss would decrease the ECFV by more than 3 L (i.e., water excretion would parallel the loss of Na+ to maintain body fluid osmolality constant: 500 mEq/day ÷ 140 mEq/L = 3.6 L/day of fluid loss). Such fluid loss in a 70-kg individual would represent a 26% decrease in ECFV.
In euvolemic individuals the nephron segments distal to the loop of Henle (distal tubule and collecting duct) are the main nephron segments where Na+ reabsorption is adjusted to maintain excretion at a level appropriate for dietary intake. However, this does not mean the other portions of the nephron are not involved in this process. Because the reabsorptive capacity of the distal tubule and collecting duct is limited, these other portions of the nephron (i.e., proximal tubule and loop of Henle) must reabsorb the bulk of the filtered load of Na+ . Thus during euvolemia, Na+ handling by the nephron can be explained by two general processes: 1.
Na+ reabsorption by the proximal tubule and loop of Henle is regulated so that a relatively constant portion of the filtered load of Na+ is delivered to the distal tubule. The combined action of the proximal tubule and loop of Henle reabsorbs approximately 92% of the filtered load of Na+ , and thus 8% of the filtered load is delivered to the distal tubule.
2.
Reabsorption of this remaining portion of the filtered load of Na+ by the distal tubule and collecting duct is regulated so that the amount of Na+ excreted in the urine matches the amount ingested in the diet. Thus these later nephron segments make final adjustments in Na+ excretion to maintain the euvolemic state.
Mechanisms for Maintaining Constant Delivery of NaCl to the Distal Tubule
A number of mechanisms maintain a constant delivery of Na+ to the beginning of the distal tubule. These processes are autoregulation of the GFR (and thus the filtered load of Na+), glomerulotubular balance, and load dependency of Na+ reabsorption by the loop of Henle.
Autoregulation of the GFR (see ) allows maintenance of a relatively constant filtration rate over a wide range of perfusion pressures. Because the filtration rate is constant, the filtered load of Na+ is also constant.